With the spread of mobile equipment, such as cellular phones and notebook computers, the role of secondary batteries, which are the power sources of the mobile equipment, is regarded as important. These secondary batteries are required to be of small size, light weight, and high capacity, and be less likely to cause the deterioration of charge and discharge capacity even if charge and discharge are repeated. As secondary batteries that satisfy such characteristics, many lithium ion secondary batteries are currently used. In addition, in recent years, a market for motor-driven vehicles, such as electric cars and hybrid cars, has taken off rapidly, and further, the development of home and industrial electricity storage systems has been accelerated. Accordingly, further development of high performance lithium ion secondary batteries has been promoted.
Carbon, such as graphite and hard carbon, is mainly used for the negative electrodes of lithium ion secondary batteries. With carbon, a charge and discharge cycle can be repeated well, but a capacity around the theoretical capacity has already been achieved, and therefore, a significant improvement in capacity cannot be expected in the future. On the other hand, the demand for an improvement in the capacity of lithium ion secondary batteries is strong, and therefore, studies of negative electrode materials having higher capacity, that is, higher energy density, than carbon are performed.
For a material that increases energy density, the use of a Li-occluding substance that forms an alloy with lithium, represented by the composition formula LiXA (A includes an element such as aluminum), as a negative electrode active material, is studied. This negative electrode active material has a large amount of occluded and released lithium ions per unit volume and high capacity.
In addition, Non Patent Literature 1 describes the use of silicon as a negative electrode active material. It is disclosed that by using such a negative electrode active material, a secondary battery with high energy density is obtained.
Patent Literature 1 describes a nonaqueous electrolyte secondary battery using a silicon oxide or a silicate containing lithium as a negative electrode active material. It is disclosed that by using such a negative electrode active material, a secondary battery with high energy density, excellent charge and discharge characteristics, and excellent cycle life is obtained.
Patent Literature 2 describes a negative electrode for a lithium secondary battery obtained by sintering under a non-oxidizing atmosphere using silicon and/or a silicon alloy as a negative electrode active material and a polyimide as a negative electrode binder. It is disclosed that by using such a negative electrode, a secondary battery with excellent cycle characteristics is obtained.
On the other hand, Patent Literature 3 describes a nonaqueous electrolyte secondary battery in which metal lithium foil previously affixed to a positive electrode is electrochemically diffused in the carbon material of a negative electrode, and lithium capable of discharging is retained in the carbon material of the negative electrode. It is disclosed that in this secondary battery, the capacity of the affixed metal lithium foil is 4 to 40% with respect to the saturation reversible capacity of the carbon material used for the negative electrode.
Patent Literature 4 describes a nonaqueous electrolyte secondary battery characterized in that lithium is allowed to previously exist in a negative electrode portion not opposed to a positive electrode. As a method for allowing lithium to exist in the negative electrode portion, affixing lithium or electrochemically doping with lithium is described. It is disclosed that in this secondary battery, the amount of metal lithium allowed to previously exist in the negative electrode portion not opposed to the positive electrode is 0.10 to 3.00 mg per 1 cm2 of the negative electrode.
Patent Literature 5 describes a nonaqueous secondary battery characterized in that the negative electrode includes a material in which SiOX (0.3≦x≦1.6) is predoped with lithium. It is disclosed that when the atomic ratio Lp of lithium released from the positive electrode and occluded in the negative electrode to the negative electrode Si, and the atomic ratio Ln of lithium with which the negative electrode is predoped to the negative electrode Si satisfy particular conditions, the energy density and the average voltage are improved, and good rate characteristics are obtained.